We hypothesize that small fractions (1-5 wt%) of cholesterol reduce the crystalline ordering of saturated lipids in lung surfactant monolayers, leading to a reduction in the shear viscosity, which enhances the surfactant's ability to flow and cover the alveolar interface. At higher concentrations, cholesterol reduces the monolayer elasticity, which in turn, leads to a decrease in the ability of the monolayer to resist collapse, leading to higher minimum surface tensions and a decrease in lung function. These hypotheses suggest an ptimal cholesterol content for a replacement lung surfactant. We will determine this optimal cholesterol content by measuring the shear viscosity and elasticity of clinical and model lung surfactants as a function of cholesterol composition using macro- and micro- rheology instruments unique to our laboratory. These mechanical properties will be correlated with isotherms, fluorescence and atomic force microscopy, and grazing incidence synchrotron X-ray diffraction to determine how cholesterol alters the molecular packing of lung surfactant lipids, which determines the mechanical properties of monolayers necessary for low surface tensions and rapid respreading and adsorption. Our goal is to determine the physiologically optimal viscosity and elasticity for rapid spreading and low surface tension and how best to achieve this optimum by controlling the cholesterol, lipid and protein fractions of a synthetic replacement lung surfactant for respiratory distress syndrome. In addition to an optimal composition, sufficient surfactant must be adsorbed to the interface from the alveolar fluid during the respiratory cycle. The lung surfactant specific proteins SP-A, B and C, along with lipids such as phosphatidylglycerol and cholesterol, are hypothesized to enhance exchange between surfactant in the subphase and the interface. However, little quantitative evidence for specific lipid and/or protein exchange exists. Also unknown is the surface pressures at what adsorption occurs, or if adsorption occurs during compression or expansion of the interface. To address this hypothesis, we will map out the three-dimensional distribution of lung surfactant components from the interface to the subphase using optical sectioning with a confocal microscope and multiple fluorescent dyes. We expect that SP-A, B and C promote adsorption;however, we do not know if specific lipids or proteins are adsorbed preferentially to the interface to optimize the monolayer composition that collapses at high surface pressures. Native SP- A, B and C will be compared to peptide mimics to evaluate the efficacy of the peptides. PUBLIC HEALTH RELEVANCE: A lack of lung surfactant, often due to premature delivery, is responsible for neonatal respiratory distress syndrome (NRDS). In 2002, NRDS affected an estimated 24,000 newborns in the US, current treatments utilize replacement surfactants derived from animals. The goal of this research is to develop an entirely synthetic replacement surfactant that should reduce costs of NRDS treatment, improve uniformity, decrease the likelihood of contamination with infectious agents, and improve the efficacy of treatment of RDS associated with meconium aspiration or acute lung injury.